In a typical stretch-leveling process for metal strip, the strip passes through a set of infeed tension rollers for applying tension and a set of outfeed tension rollers for relieving the tension. In a first process step, the strip is alternatingly bent between the set of infeed tension rollers and the set of outfeed tension rollers around individually supported (that is to say, not frictionally interconnected) stretch rollers so as to be increasingly stretched. Then in a downstream step, relative to a strip-travel direction, the strip passes through a multiroller straightening set downstream of the last stretch roller and having a plurality of straightening rollers around which the strip is bidirectionally bent for reducing strip curvature and/or residual tension. The extent to which the strip (overall) is plastically stretched and therefore elongated is called the stretch ratio.
With a stretch-leveling method such as this, nonplanar metal strip can be leveled and thus, nonplanarities can be eliminated. Nonplanarity means, for example, strip waviness and/or strip camber that come about as a result of differences in length of the strip fibers in the strip plane. However, nonplanarity also means strip curvatures longitudinally and/or transversely that come about as a result of bending moments in the strip, for example, when the strip was bent elastic-plastically around deflection rollers, and/or due to deformations during winding of the strip. Longitudinal curvature is also referred to as coil set, transverse curvature is also referred to as cross bows. In the process of stretch-leveling, the nonplanar strip is bent (bidirectionally) while under tension that lies below the elastic limit of RE, that is, the technical elastic limit RP 0.01 thereof of the strip material, around rollers of sufficiently smaller diameters that by the combination of tension and bending, an elastic/plastic deformation is produced in the strip. The strip is plastically elongated and the level of the plastic elongation is referred to as stretch ratio. During plastic elongation, the originally short strip fibers are elongated to a relatively stronger degree. Ideally, all strip fibers are of the same length after being leveled so that basically a perfectly leveled strip free of waviness or strip cambers is produced.
With the prior-art stretch-leveling method, residual bending moments can remain in the strip after leveling due to the bidirectional bending in the elastic-plastic region that can become visual as transverse curvature in the strip treatment line, and in a cut-out plate can lead to a longitudinal and/or transverse plastic residual curvature. The residual bending moments come about when the individual bends are not coordinated with one another in their intensity. The bending radii depend on the strip data (thickness, elasticity module, cyclic strength behavior, Poisson's ratio), the strip tensile stress, the diameter of the roller, and the geometry of the strip path around the rollers. In a first approximation, the latter can be described by the wrap angle of the strip around the rollers. With a sufficiently large wrap angle or sufficient tensile stress, the strip assumes the radius of the roller. The strip curvature then reaches its maximum and remains constant with increasing wrap angle or increasing tensile stress. However, as a rule, the wrap angles are adjusted such that the strip does not follow the roller radius.
Even with an optimal adjustment of a predetermined stretch-leveling stand, residual bending moments occur due to fluctuations of the process parameters. For in practice, both the tensile stress, and consequently the stretch ratio as well as the mechanical strength properties and the strip thickness of the strip are basically subject to certain fluctuations.
The simple stretch-leveling process with only three bends reacts relatively sensitively to fluctuations in the mentioned parameters. Therefore, depending on application, unacceptably high residual curvatures remain in the strip.
A distinct improvement is brought about by a stretch-leveling process with four individual straightening rollers that can be set up individually (see US 2012/0174643).
There are higher requirements, for example, if the stretch limit and the required stretch ratio become very high, for example, with high-strength steel strip. More bends may then be required in order to keep the residual curvature values within smaller values, for example, smaller than 10−1 (1/m).
The conventional solution, with which a plurality of bends can be used is a multiroller straightening set having, for example, a fixed lower cassette or an adjustable upper cassette with straightening rollers and backing rollers. As a rule, the upper cassette can be vertically positioned, and can be adjusted with respect to its inclination. In this manner, wrap angles decreasing linearly from roller to roller can be achieved, with the first and the last straightening roller each only having half the wrap angle and primarily functioning as pass line rollers. The disadvantage of such a conventional solution is that within a cassette the straightening rollers are frictionally coupled to one another by the backing rollers, that is to say, all the straightening rollers run at the same speed. However, due to the elongation (that is, the stretch ratio) generated in the strip during the stretch-leveling, the strip accordingly runs faster and faster from roller to roller. As a result, slippage occurs in the multiroller set with the resultant to vibrations that can lead to undesired chatter marks on the strip. The risk of chatter marks increases with the stretch ratio so that in practice, this type of design is mostly used with low stretch ratios.
For these reasons, it has already been proposed to combine conventional stretch-leveling stands with multiroller straightening units. Such a stretch-leveling method of the type as described above is known from U.S. Pat. No. 5,666,836. The roller settings are adjusted using a control that works using a mathematical model with or without feedback. The stretch ratio is thereby generated by stretch rollers that can operates as pairs. In the multiroller set, only the correction of longitudinal curvature errors then takes place without substantially increasing the stretch ratio.
The advantages of a combination of a stretch-leveling stand having a multiroller straightening unit have already been described in the technical articles “Advances in Leveling Machines” by Keiji Yamamoto and Keizo Abe in “Journal of the Research Group of Flattened Metal Technology 31 (1992), 24-31”. There, the stretch ratio is also produced with stretch rollers that are set up in pairs. The correction of curvature errors then takes place in the multiroller set.